When enlisting another minion you get the choice between 2 prices (except when you have a base character)

* Choosing the top option, the most expensive one, the new minion will get 1 experience for every $3 you pay.
* Choosing the bottom option, the less expensive one, the new minion will get 1 experience for every $5 you pay.

Thanks, I looked but couldn't find it initially, forgot there was a search feature haha :D